Kinetic model of GPCR-G protein interactions reveals allokairic modulation of signaling

Kelly J Culhane,Indrani Madhugiri,Chetan J Gadgil,Sivaraj Sivaramakrishnan,Tejas M Gupte

doi:10.1038/s41467-022-28789-5

Abstract

Established models of ternary complex formation between hormone, G protein coupled receptor (GPCR), and G protein assume that all interactions occur under equilibrium conditions. However, recent studies have established that the lifetimes of these interactions are comparable to the duration of hormone activated GPCR signaling. To simulate interactions during such non-equilibrium conditions, we propose a kinetic model wherein the receptor undergoes rate-limiting transitions between two hormone-bound active states. Simulations, using experimentally measured parameters, demonstrate transient states in ternary complex formation, and delineate the phenomenon of GPCR priming, wherein non-cognate G proteins substantially enhance cognate G protein signaling. Our model reveals that kinetic barriers of slow receptor interconversion can be overcome through allokairic modulation, a regulatory mechanism of ternary complex formation and downstream signaling.

Highlights

Established models of ternary complex formation between hormone, G protein coupled receptor (GPCR), and G protein assume that all interactions occur under equilibrium conditions
Our model reveals that Qpep acts a positive allokairic modulator, which increases cognate ternary complex formation by overcoming the rate limiting conformational transition between receptor interaction states (HR’ → HR*)
We have previously shown that effectors that bind at the G protein binding site, including the non-cognate Gαq-peptide (Qpep), can serve as positive allokairic modulators of the GPCR, by priming the receptor for increased signaling[13,22]

Summary

Introduction

Established models of ternary complex formation between hormone, G protein coupled receptor (GPCR), and G protein assume that all interactions occur under equilibrium conditions. While the agonist bound receptor alone rarely transitioned to its fully active state (~1 min−1), the presence of the G protein enhanced this transition rate to 0.1–0.7 s−1 in an agonist efficacy dependent manner[11] These data suggest kinetic barriers between receptor conformational states (HR’ → HR*; Supplementary Fig. 4) that mirror the slow movements in the β2AR transmembrane domains in response to some agonists (~1–3 min20) and the metastable active conformational states (lifetime of ~1 min) observed in rhodopsin following ligand dissociation[12]. Despite these advances, a detailed kinetic model that allows incorporation of the effects of non-cognate G protein or other effectors that interface with the receptor is still lacking and forms the emphasis of our study. Our integrated kinetic modeling and experimental data highlight the importance of understanding the timescales of interactions in GPCR signaling pathways, while establishing allokairic regulation in ternary complex formation and downstream signaling

Methods

Results

Conclusion